A Review of Non-Chemical Weed Control
Techniques

S. Parish

The West of Scotland College, Auchincruive, Ayr, KA6 5HW,
Scotland*

ABSTRACT

Weeding has traditionally been a labour intensive operation in
crop production. The use of herbicides was rapidly accepted by
many farmers and became an accepted part of crop husbandry,
although a few farmers always questioned the widespread use of
chemicals in farming, and the concept of organic farming
necessitated a non-chemical approach to weed control.

The recent upsurge in environmental awareness of the public,
interest in organic food production and some problems with
herbicide use, has led to a range of techniques and machines
being developed for non-chemical weed control. Thermal and
mechanical techniques are reviewed for cereal and row crop
production.

INTRODUCTION

Ever since the first cultivation systems were developed for
food production farmers of all generations and areas have been
faced with the problems of noncrop plants growing amongst the
crops. These non-crop plants, which compete with the crops for
moisture, light, nutrients and space, have long been known as
weeds.

The problems which these non-crop plants have caused to
farmers have led to the term weed being used as an insult to
other humans, often inferring lack of courage or strength. Yet
weeds which are thin, spindly and pale are often so because of
their resilience and ability to compete with the crop plants. The
trade names of herbicides developed to control weeds imply that
they are a challenge worth combating, or names such as Avenge,
Crusader, Harrier and Stomp would not be used.

With the much greater public awareness of food and
environmental issues which has developed rapidly in recent years,
it is probably worth looking at weed control from a wider
perspective, and particularly at methods of weed control in
systems where herbicides are too expensive or ineffective to use
and in systems in which they are not permitted for use.

A weed can be thought of as any plant growing in the wrong
place at the wrong time. In crops weeds can cause problems of
severely reduced yields and also affect the efficient use of
machinery, for example in harvesting and crop storage. Effective
weed control is therefore an important part of crop husbandry,
and has traditionally been a labour intensive operation. In the
1880's a large farm of 100ha near Glasgow had a regular staff of
3 or 4 ploughmen and 2 female servants. In April 20-24 extra
women were hired for the planting of potatoes and in late May
30-40 workers were taken on to weed the potato crop (Orr, 1984).
In less developed countries the situation still exists where the
peak labour requirement is often for hand weeding (and still done
by women (Rogers, 1979)). If this labour demand cannot be met,
then the crop must be grown on a smaller area than would
otherwise be economically viable.

WEED CONTROL AND ENVIRONMENTAL ISSUES

As agriculture became mechanised cultivation techniques for
weed control were developed, particularly for inter-row work in
widely spaced row-crops. A significant amount of manual work was
still required for the weeds in the crop rows, although steerage
hoes were used for the inter-row areas. To control couch
(Agropyron repens) and other rhizomatous weeds, intensive
cultivations were practiced, particularly using "L"
blade rotary cultivators (Fail, 1956). Inter-row cultivations for
weed control in potatoes can cause problems of clod formation and
variations on the traditional equipment design were developed
(Green, 1962).

The huge reserve of weed seeds in the soil means that any
cultivation operation will stimulate another flush of weeds to
germinate. The nature of the growth of the crop plants then
becomes an important factor. Leafy growth which spreads to cover
the ground can effectively smother the weeds but crops such as
onions, which have thin leaves tending to stay well above the
soil surface, are prone to severe weed competition.

It is therefore hardly surprising that the development of
chemicals to act as herbicides gained rapid acceptance among
farmers, as successful weed control is a major contribution to a
successful crop. Extremely toxic substances, such as sodium
arsenite and DNOC, one of the first selective herbicides, were
succeeded by translocated and growth affecting chemicals like
MCPA, which is less toxic to animals. Persistent and residual
herbicides, such as simazine and the longer lasting linuron, are
now available for appropriate applications. However, in the
1960's concern was already being shown over the environmental
effects of pesticides in general, and insecticides in particular.
Herbicides were also investigated, and studies in the USA as long
ago as 1959 showed aminotriazole, a translocated weedkiller, to
be carcinogenic (Carson, 1965).

The use of arsenical weedkillers to destroy potato haulm
ceased in the UK after 1961 but even herbicides which are less
toxic to animals continue to cause concern. The wide use of MCPA
since the 1940's has led to a significant decline in numbers of
wild flowers (Mellanby, 1969), and although many will regard the
term "wild flower" as a euphemism for "weed",
even these have their uses, particularly from a medicinal or
dietary point of view. For example, stinging nettle (Urtica
dioica) can be used as a diuretic and to improve the blood
supply; common fumitory (Fumaria officinalis) can aid digestion
and scented mayweed (Matricaria chamomilla) is used for skin
conditions (Kresanek, 1982). Where floating weeds, such as the
water hyacinth (Eichhornia crassipes) have become a nuisance,
they can be harvested and used as a fertiliser source or pig feed
(Pirie, 1978).

The diversification of weed communities has recently been the
subject of several studies, particularly those comparing weed
species between organic and conventional fields and the field
edge and field interior. Van Elsen (1990) found that
biodynamically cultivated grain fields show a wider diversity of
species with less contrast between the interior and edge, whereas
the grain fields treated with herbicides have more weed species
at the edge (Table 1). Plakolm (1990) also found that in
conventional field boundaries there are more species only present
there, which are not present in the field interior.

In 1988 the British Crop Protection Council initiated the
organisation of a symposium in order to bring together ideas on
how to develop reliable methods for identifying and evaluating
the effects of pesticides on the environment. In the paper
presented by Booij & Noorlander (1988), it was suggested that
the effect of herbicides on fauna is primarily influenced by the
availability of soil cover and therefore lack of prey. However,
it was acknowledged that further investigations are needed to
study the effect of herbicides on the ecology of cropping areas.

Environmental issues, therefore, are a significant influence
on the development of non-chemical weed control techniques. There
are, however, more mercenary aspects to consider. The high cost
of herbicide treatment in crops such as sugar beet and fodder
beet have led to investigations into reduced herbicide use, such
as band spraying along the crop rows combined with inter-row
cultivations. Some farmers or growers may be tempted by high
prices to consider organic production, in which no herbicides are
permitted (UKROFS, 1989) and the reliance for weed control
reverts to mechanical, thermal, cultural and management
practices. Other horticultural crop producers may, through poor
management, find themselves with a weed spectrum for which there
is no effective herbicide permitted for application to the crops
being grown (Haas & Streibig, 1982).

WEED COMPETITION

At this stage it is worth considering some basic aspects of
weed management, before looking in detail at the techniques
available for non-chemical weed control. An awareness of the
common weeds in the different fields is important, so operations
such as cultivations, sowing and weeding can be timed according
to the peak germination periods of the predominant species
(Figure 1). In continuous cereals the range of weeds is often
reduced to those whose cycles fit with that of the cereal crop.
For example, if cleavers (Galium aparine) are a major problem,
then the sowing of winter cereals should be delayed until after
their peak time for germination. Crop rotations should be
designed so that the differences in the timing of seedbed
cultivations prevents one weed species becoming dominant
(Lockhart et al., 1990).

The total absence of weeds has only become a possibility
following the introduction of herbicides. However, the complete
removal of weeds from within a crop may itself cause other
problems. Insects then have no alternative but to attack the crop
itself and there is no suitable cover for the predators of crop
pests (Altieri & Letourneau, 1982). Agronomists and
statisticians have yet to agree on the effect on crop yield, or
the cost/benefit analysis, of the presence of low weed densities,
as is indicated by the conflicting curves shown in Figure 2
(Cousens, 1987). The situation in organic crops has yet to be
researched and the beneficial effect of weed presence is a topic
outside the mainstream of agricultural research and development.
No doubt the 1990's will see more investigations in this area.

Combined with the knowledge of weeds themselves, an
understanding of the tolerance of crops to weed competition can
be critical in establishing the timing of weeding operations. The
timing of weeding in vegetables has been shown by Weaver (1984)
to be critical in order to avoid yield reduction, and the
duration of weed competition is also influential. For most crops,
a critical period exists during which weeds must be controlled to
maintain yields. The results of experiments in onions show that,
during the first four weeks after 50~0 emergence, the weeds had
little effect on yield arid weeds allowed to grow on after ten
weeks also did not reduce crop yield (Figure 3). The critical
period of weed competition for this crop is therefore from four
to ten weeks. The onion crop is particularly difficult to keep
weed free, other than by hand or with herbicides, although
thermal techniques for the crop are outlined later in this paper.

Tables on other crops, for the toleration of early weed
competition without yield loss and for the weed-free period
required to prevent yield loss, are published in an international
review by Zimdahl (1980).

NON-CHEMICAL WEED CONTROL

It is extremely difficult to put a price on the research and
development costs of the herbicides used in agriculture today. It
would not be a gross exaggeration to state that less than 1% of
that cost has been spent on the development of non-chemical weed
control methods and yet the major food retailers expect their
sales of organically produced vegetables to be at least No of
total sales by the mid-1990's. Non-chemical weed control does not
necessarily imply reverting to outdated techniques and an
impressive array of modern machinery already exists, some of
which are new ideas and others developments of more traditional
implements. The role of these machines for effective weed control
should now be examined as part of a weed management strategy.

Weed control in organic cereals

A good rotation is needed for pest and disease control, the
maintenance of soil fertility, as well as for weed control. An
example of a mixed farm 8 year rotation is:

3 years grass/clover ley

2 years winter wheat

1 year arable silage (cereals/legumes)

1 year potatoes

1 year potatoes 1 year spring barley (undersown)

The 3 year ley is to control annual weeds, while undersowing
helps to smother weeds and provide soil cover in the winter.

The timing of sowing is important, and with winter wheat,
autumn weed problems can be avoided by delaying sowing until late
October or November. A cereal variety with long straw and an
initially prostrate growth habit, which covers the ground
quickly, enables the crop to smother the weeds. However, in
organic production resistance to common diseases is a
prerequisite for variety selection (Parish, 1989a). Other
techniques to smother weeds include increasing the sowing rate by
up to 20%, sowing cereals in bands and, as in parts of mainland
Europe, undersowing winter cereals with mustard (Lampkin, 1986).

Higher seed rates are desirable, not only to provide more
competition for weeds but also to help compensate for any crop
loss due to weed control cultivations after sowing. A thin-tined
implement (Figure 4) can be used for operations pre- and
post-emergence. Blind harrowing, just before the crop emerges,
aims to disturb weeds which have already germinated. Careful
examination of the soil to examine the weed seeds is essential,
otherwise germination of the weeds will be stimulated instead.
Once the crop has reached the 3 leaf stage harrowing can be
effective, with chain harrows or striegels being used at speeds
up to 8 km/h. The draft requirement is low, and a workrate
similar to that for herbicide application can be achieved for a
similar effect at a reduced cost (Brautigan, 1990). Some harrows
have adjustments for different levels of pressure on the soil, to
match the prevailing conditions. Hoeing cereals is possible if
the rows are spaced closer than normal drills allow, in bands,
and between the bands a wider gap is left for a hoe (Figure 5).
The overall sowing rate should be increased by 10% for this
system. Mustard sown as a fast growing cover crop to smother
weeds in autumn sown cereals is possible if severe frosts are
certain to kill the mustard, to prevent it competing with the
crop in the spring. It is also thought that mustard may have an
allelopathic effect on weeds.

Further investigations into the natural enemies of weeds are
likely to identify either insects or diseases which can be used
as a form of biological control. Examples here are the effect of
ground beetles on weed seeds in USA maize production and the use
of the rust Puccinia chondrillina to control the weed Chondrilla
juncea L, which had become a pest in Australian wheat and pasture
areas (Andres & Clement, 1984).

In a report by Patterson & Bufton (1986), the modification
of combine harvesters was recommended in order to separate the
weed seeds from the grain, straw and chaff to avoid returning
seeds to the soil. The destruction of weed seeds in the soil has
been found possible by the use of microwaves which distort the
molecules of tissues and cause lethal internal damage (Davis,
1975). However, for the control of wild oat (Avena fatua) seed a
similar method was found to be impractical for field use due to
high energy costs (Lal & Reed, 1982).

Weed seeds can also be returned to the soil through animal
wastes. To reduce the viability of seeds, the wastes need to be
composted and to reach a temperature of at least 60°C (Soil
Association, 1989). In UK conditions aerated slurry lagoons are
unlikely to reach this temperature without a cover and insulated
sides.

Weed control in organic row crops

Many of the aspects of weed management already discussed are
also pertinent for row crops. A wide range of machines exist for
weed control in row crops and it is useful to consider the two
major types separately.

Thermal techniques for weed control

The control of weeds in the crop row is a major problem in
many organic crops. To combat this problem, thermal techniques
pre-emergence of the crop are becoming more sophisticated and
there are also some crop plants which can tolerate post-emergence
treatment at specific growth stages.

Thermal techniques, often called flame weeding, generally use
liquified petroleum gas (LPG), mostly propane, but in the 1950's
work was also carried out using oil burners to reduce weed
competition in bulb production (Wolfe & Horton, 1958).
However, some experimental work has been carried out into the use
of electricity, both as a heat energy source and for electrical
shock treatment. Of the latter, two methods, spark discharge and
electrical contact, are under development, both needing voltages
of around 20 kV to be effective. An electrical contact machine to
control sugar beet bolters has produced a plant kill rate of 40%,
compared to 60% for chemical control (Diprose et al., 1985). Only
one machine of this type has been produced commercially and its
capital cost makes its use uneconomical below 900 ha/annum.
However, compared with chemical use there are no dangerous
residues following the treatment and therefore no delay in
subsequent operations. Also the field efficiency of the operation
is high, as there is no requirement to refill sprayer tanks or,
in the case of thermal treatments, exchange gas cylinders.

The high voltage required for these machines poses a hazard,
which may be less of a problem if lower voltages were used to
generate heat to expose weeds to infra-red radiation. Laboratory
investigations into the effects of different infra-red
wavelengths on plants, identified a medium wave tubular fused
quartz emitter to be the most effective of the infra-red emitters
tested. Energy intensities between 200 and 400 kJ/m2 were
required to severely affect plant growth at the seedling stage
(Parish, 1989b), with dicotyledons more susceptible to heat than
monocotyledons.

LPG fuelled flame weeders have now become established as part
of the organic grower's machinery complement. The aim of a flame
weeding operation is not to burn off the weeds but to apply
sufficient heat to severely damage the plant cells so the plant
will eventually wither and die. The technique involves raising
the plant tissue to a temperature of 100°C for 0. is, in order
to burst the cell membranes. Coagulation of proteins occurs
between 50°C and 70°C (Hoffmann, 1980). The accurate
measurement of temperatures of small plants for short time
periods is not easy, although some methods have been documented
and a workshop to discuss techniques was held in February 1990 in
Denmark. Two basic designs of flame weeder are available, flame
contact and infra-red. The latter design (Figure 6) involves
heating a surface to radiate energy on to the plants being
treated. The heated surface is required to be about 1.5 m in
length, as exposure of plants to the heat for approximately 2
seconds is required. Infra-red equipment has a much higher
purchase price, but was considered to be more economical on gas
when compared to the flame-gun type shown in Figure 7. Recent
trials have shown the infra-red performance to be inferior to
that of flame guns (Ascard, 1990), but the latter design is
continually being refined to improve the efficiency of gas use.
Shielded burners, which utilise more of the energy of flame
combustion, are now being developed and have both a contact and
radiant effect on the weeds being treated. Gas energy use has
been recorded within the range 8 to 36 kg/ha, according to trials
on carrots by Vester ( 1984). Other applications of flaming are
for onion and potato haulm destruction.

There is very little evidence of research carried out on the
integration of burners into flame weeder design. An experimental
approach has been taken to investigate the effect on plants of
the burner position, for different flame treatments (Parish
1989c, 1990). The effect of burner design, speed of travel,
height above the ground and angle to the ground were also
investigated, along with the effect of gas pressure and direction
of travel. Some results are presented in Table 2.

The use of LPG to control weeds is seen in some quarters as
contrary to one of the basic principles of organic production, in
that the reduction in use of fossil fuels is seen as a major
objective (IFOAM, 1981). Refining the design of flame weeders,
and improving their efficiency in use, goes some way towards
achieving this objective. Table 3 shows how the energy intensity
of a single burner can vary according to use.

Crops, such as carrots and parsnips, which have a long
germination period can be flamed pre-emergence. From the date of
sowing to emergence is normally at least 10 days and within this
period many weed seedlings can emerge, having been stimulated
into germination by the seedbed preparation operations. The
timing of the operation is crucial for successful weed control.
The ideal time is just prior to the emergence of the carrot crop
and the best way to determine this is by very careful examination
of the sown seeds. Unless the relative cost of LPG is low, a band
operation is usually carried out with the aim being to treat an
effective width of at least 100 mm. (Weeds between the rows can
be controlled by subsequent inter-row cultivations). It is
difficult to visually assess the effect on the weeds immediately
after the flame has passed.

A guideline is to press a treated leaf lightly between a
finger and thumb, and a lasting imprint on the leaf indicates
extensive cell damage. If it is considered that a greater effect
is required, this can be achieved by either increasing the gas
pressure or reducing the travel speed. Typical operating
parameters are: propane gas used between 1 and 2 b pressure,
burners 100 to 200 mm above the ground set at an angle of 30-45°
to the horizontal (Parish, 1987a). Speed of travel is slow, 2-4
km/in, which gives a work rate range of 0.4 to 0.8 ha/in for a
machine matching a 2 m bed system.

Crops such as maize and onions can tolerate flaming
post-emergence, at specific growth stages. In the Federal
Republic of Germany flaming and inter-row cultivations are
techniques widely used in maize production, as there is an area
of some 75000 ha where the weeds have become resistant to
atrazine and there are traces of the chemical being found in
drinking water (Hoffmann 1990). The requirement for hand weeding
onions is significantly reduced by flame weeding, particularly in
set onions, for which flaming at three stages is possible. At a
crop height of 5 cm, the burner was set directly above the plants
(Figure 8), whereas at crop heights of 20 and 40 cm, flaming was
done with burners set at each side of the row, fitted with leaf
lifters and leaf protectors (Ascard, 1990).

Inter-row cultivations

There are a large number of different designs of inter-row
cultivators available on the European market, varying from
traditional spring-tine cultivators to novel pto-powered brush
type machines. The basic requirements for successful inter-row
cultivations for weed control are:

i) to cut or uproot weeds, and then either completely bury
them or leave them on the soil surface for desiccation;

ii) to protect the crop plants;

iii) to control implement direction; iv) to control implement
depth;

v) to maintain or improve soil conditions (Parish, 1987b).

Although inter-row cultivations are normally used to control
weeds between the crop rows, some investigations have been
carried out to control weeds in the row. Klooster (1982) found
that setting the implement to direct soil into the crop row to
cover small weeds was as satisfactory as herbicide use.
Mechanical gappers and thinners, widely used before herbicide
development, have yet to come back into favour for weed control
in the rows. In the development of a new hoe ridger, experiments
on plants grown in trays showed a 90% kill rate in dry
conditions, 57% by incorporation and 33% by desiccation (Kerpstra
& Kouwenhoven, 1981). This hoe ridger (Figure 9) was designed
for use in the later stages of sugar beet growth, and a similar
effect is claimed for the ground driven rolling cultivator
(Figure 10). This versatile machine can be operated at speeds up
to 12 km/in and set to direct soil into the crop rows, to ridge
soil, to cut down ridges, or simply to disturb weeds between the
rows. The blade design produces a cutting and mixing effect, but
Mattson (1990) found that the depth of work was insufficient to
control larger weeds and that the correct setting and operation
of all inter-row cultivators influenced the results.

Steerage options are available for rolling cultivators, brush
weeders and fined cultivators. The advantage is to be able to
work closer to the crop rows without causing plant damage, but
the effect of soil disturbance close to the root zone has yet to
be quantified in terms of plant growth. The design of guards used
to protect the crop leaves can vary from rotating discs to
floating shields or tunnels. The crop needs protecting both from
soil thrown by the tines and from the tines themselves.

To avoid crop damage, the cultivator must be accurately set to
match the drills and to run straight. Rear-mounted machines
should be operated with the tractor check chains slack and the
top link exactly in line with the direction of travel. Any
steering deviation of the tractor is then not passed on to the
cultivator. Mid-mounted machines, as attached to tool-carrier
tractors follow the direction of steering best, and front
mounting is preferable to rear mounting for directional control.
The blade depth of a spring tine cultivator is maintained through
a depth wheel connected to a parallel linkage mechanism.

These components, together with the steering disc which
prevents lateral slippage and helps maintain direction, and crop
protectors are clearly shown in Figure 11.

The tine design can either be a rigid sweep-tine, which works
below the soil surface and cuts the weed stems away from the
roots, or a spring-tine with a goosefoot share. The vibrating
action of the spring-tines disturbs the soil, lifting out the
weeds. Three tines are usually used, in a delta formation between
the crop rows, with the middle share overlapping the outer ones.
The recommended speed of work is up to 10 km/in for some models.

The pto-powered brush weeder (Figure 12) consists of flexible
polypropylene brush discs, assembled into units of the desired
width and spacing for the crop onto a horizontal pto-driven
shaft. The brushes are set to work at a depth of 50 mm, and the
crop rows are protected by tunnels 600-800 mm long and of the
appropriate width, usually 40-200 mm. The effect of the brushing
action is to lift the weeds out of the soil, causing damage by
stripping leaves and breaking stems. The roots are brushed bare
of soil, leaving the weeds far more vulnerable to desiccation.
Compared to the other designs of inter-row cultivator the brush
weeder can be effective in wet conditions. In dry conditions the
other machines would be more appropriate, due to their faster
work rates and lower running costs. The effects of these machines
on soil surface structure are not well documented.

Pederson ( 1990) has defined the intensity of work of the
brush as the ratio of the brush peripheral speed to the forward
speed of the machine. The best effect on weeds was obtained with
a work intensity of about 6, and the effects of a range of travel
speeds, for the same brush speed, were investigated in weeds at
different growth stages (Table 4).

Other techniques

Light exclusion techniques are widely used in small scale
horticultural organic crop production. Materials used include
black plastic, carpet, straw, cardboard, tree bark and wood
chippings. The effects of such mulches on weeds, pests and crop
yields, have recently been studied by several workers. In their
study, Lennartsen et al. (1990) compared different surface
mulches for their effect on clearing an established grass pasture
and subsequent crop yields for organic horticultural produce.
Weibel & Niggli (1990) investigated available nutrient levels
in the soil, yields and fruit flavour in apple orchards.

CONCLUSIONS

Techniques for non-chemical weed control have been developed
to reduce chemical costs in conventional agriculture, in response
to environmental pressures and to provide for the needs of
organic food production. A wide range of equipment is available
to cover the major crops grown. Successful non-chemical weed
control requires a well managed, integrated system and attention
to detail.

Future work is required to research the effects of heat from
thermal techniques on soil micro organisms, and weed seed
germination and viability. The effects of the different
soil/weed/tine combinations on the success of the weeding
operation and on the soil structure also merit attention.

ACKNOWLEDGEMENTS

The author would like to thank Nic Lampkin for the opportunity
to read the manuscript of the Weed Management chapter of his
book, Organic Farming, which is due to be published in 1990 by
Farming Press.

Ascard, J. (1990). Thermal weed control with flaming in
onions. In Proceedings of the Third International Conference on
Non-chemical Weed Control held in Linz, Austria, October 1989 (G.
Plakolm, ea.) In print.

Brautigan, V. (1990). Mechanical weed control in grain with
finger weeder and chain harrow in areas with conventional and
reduced tillage. In Proceedings of the Third International
Conference on Non-Chemical Weed Control held in Linz, Austria,
October 1989(G. Plakolm, ed.) In print.

Haas, H. & Streibig, J.C. (1982). Changing patterns of
weed distribution as a result of herbicide use and other
agronomic factors. In Herbicide Resistance in
Plants(H.M.LeBaron&J.Gressel, eds.), pp. 57-59. John Wiley
& Sons; New York.

Hewson, R.T. & Roberts, H.A. (1971). The effect of weed
removal at different times on the yield of bulb onions. Journal
of Horticultural Science, 46, 471-483.

Hoffmann, M. (1980). Abflammtechnik. KTBL; Darmstadt.

Hoffmann, M. (1990). New experiences in thermal treatment of
Indian corn and potatoes. In Proceedings of the Third
International Conference on Non-chemical Weed Control held in
Linz, Austria, October 1989 (G. Plakolm, ea.) In print.

Klooster,J.J.(1982).The role of tillage in weed control. ln
Proceedings of the 9th Conference of the Soil Tillage
Research Organisation. Institution of Agricultural Engineering,
Wageningen. Pp. 256-261.

Lennartsson, E.K.M. et al. (1990). The use of light exclusion
techniques for clearing grass pasture in organic horticultural
systems. In Proceedings of the Third international Conference of
Non-chemical Weed Control held in Linz, Austria, October 1989 (G.
Plakolm, ea.) In print.

Lockhart, J.A.R. et a (1990). The evolution of weed control in
British agriculture. In Weed Control Handbook: Principles (R.J.
Hance and K. Holly, eds.), pp. 43-74. Blackwell; Oxford.

Mattson, B. (1990). Investigation of seven inter-row weeders.
In Proceedings of the Third International Conference of
Non-chemical Weed Control held in Linz, Austria, October 1989 (G.
Plakolm, ea.) In print.

Parish, S. (1990). A procedure for assessing flame treatments
under controlled conditions. In Proceedings of the Third
International Conference on Non-chemical Weed Control held in
Linz, Austria, October 1989 (G. Plakolm, ed.). In print.

Patterson, D.E. & Bufton, L.P. (1986). Report on organic
food production in the UK and the scope for engineering
development, ON 1377, AFRC Institute of Engineering Research;
Silsoe.

Pederson, B.T. (1990). Test of the multiple row brush hoe. In
Proceedings of the Third International Conference on Non-chemical
Weed Control held in Linz, Austria, October 1989 (G. Plakolm,
ea.) In print.

Plakolm G. (1990). Studies of weeds on organically and
conventionally farmed grain fields in Upper Austria. In
Proceedings of the Third International Conference on Non-chemical
Weed Control held in Linz, Austria, October 1989 (G. Plakolm,
ed.). In print.

Van Elson, T. (1990). Weed communities at the edges and in the
interior of grain fields and root crops cultivated in different
ways. In Proceedings of the Third International Conference on
Non-chemical Weed Control held in Linz, Austria, October 1989 (G.
Plakolm, ea.) In print.

Vester, J. (1984). New experience with flame cultivation for
weed control. In Proceedings of the International meeting on
flame cultivation for weed control, held in Namur, Belgium,
November 1984 (C. Castille, ed.). pp. 10-20. CRABE; Opprebais.

Weibel, F.B. & Niggli, U. (1990). Weed control in apple
orchards by organic soil covers. In Proceedings of the Third
International Conference on Non-chemical Weed Control held in
Linz, Austria, October 1989 (G. Plakolm, ea.) In print.